Dutch roll and spiral instability
Two outcomes of dihedral vs directional stability imbalance.
| Dutch roll | Spiral instability | |
|---|---|---|
| Imbalance | Dihedral (lateral) effect stronger than directional stability | Directional stability strong relative to dihedral |
| Motion | Oscillation | One-way divergence |
Dutch roll (PHAK ch.5):
- Coupled roll and yaw oscillation, out of phase: nose traces figure-eight on horizon. Not pitch or CG effect, not yaw alone.
- Usually damps out on its own in a few cycles, unless air stays gusty or turbulent.
- Swept, high-speed types with continuing tendencies usually fitted with gyro-stabilized yaw damper (PHAK ch.5).
- Bigger fin does not fix it: fin is the stability source.
- More dihedral pushes balance toward Dutch roll, does not damp it (PHAK ch.5).
Spiral instability:
- Gust causes sideslip.
- Strong fin yaws nose into relative wind faster than weak dihedral levels wings.
- Outside wing gains lift, bank steepens, nose drops.
- Left uncorrected: steep spiral dive. Does not self-correct when gust ends.
- Wing leveler is the aid for spiral instability (PHAK ch.5).
References
- PHAK ch.5, Free Directional Oscillations (Dutch Roll); Spiral Instability